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Function RTKSetTaskSwitchHook

RTKernel-32 can call a user-defined function in every task switch. Such a task switch hook is installed using function RTKSetTaskSwitchHook:

typedef void (__cdecl * RTKTaskSwitchHook)(RTKTaskHandle OldTask, 
                                          RTKTaskHandle NewTask); 

void RTKSetTaskSwitchHook(RTKTaskSwitchHook   Hook, 
                          RTKTaskSwitchHook * OldHook);

Parameters

Hook

A function with two task handles as parameters, respectively referencing the task being suspended and the task being activated.

OldHook

Should point to a variable to receive a pointer to the previously installed hook.

For an application task switch hook, some important restrictions apply. Specifically, it must be considered that:

• Interrupts are disabled when the hook is called. Interrupts must never be enabled inside the hook.
• The stack context is undefined. There might be very little stack space available.
• Except for functions RTKGetUserData and RTKSetUserData, no RTKernel-32 API functions may be called.
• Before returning, the hook must call the previously installed task switch hook.

If any of these restrictions is violated, the program will very probably crash. Possible error messages may be misleading.

If Win32 emulation is used, all TLS-related functions will access OldTask's data.

Using a task switch hook is discouraged. In particular, it should not be attempted to solve reentrance problems by using a hook; resource semaphores are much better suited for this purpose. It should also be noted that a task switch hook can severly degrade RTKernel-32's performance.

The following example shows how the number of task switches can be counted for each task using a task switch hook:

#include <stdlib.h> 
#include <stdio.h> 
#include <rtk32.h> 

int UserDataIndex; 
RTKTaskSwitchHook OldHook; 

void RTKAPI MyThread(void * p) 
{ 
   while (1) 
      RTKDelay((RTKDuration)p); 
} 

void RTKAPI TheHook(RTKTaskHandle Old, RTKTaskHandle New) 
{ 
   int Count; 

   Count = (int) RTKGetUserData(New, UserDataIndex); 
   Count++; 
   RTKSetUserData(New, UserDataIndex, (void*) Count); 
   OldHook(Old, New); 
} 

int main(void) 
{ 
   RTKTaskHandle H1, H2; 

   RTKernelInit(3); 
   UserDataIndex = RTKAllocUserData(); 
   RTKSetTaskSwitchHook(TheHook, &OldHook); 
   H1 = RTKCreateThread(MyThread, 2, 1024, 0, (void*)1, "ThreadA"); 
   H2 = RTKCreateThread(MyThread, 2, 1024, 0, (void*)3, "ThreadB"); 
   RTKDelay(20); 
   printf("Task switches for ThreadA: %i, ThreadB: %i\n", 
           RTKGetUserData(H1, UserDataIndex), 
           RTKGetUserData(H2, UserDataIndex)); 
   return 0; 
}

Function RTKSetMessageHandler

Function RTKSetTaskStartStopHook